Geographic Range

Onchocerca volvulus can be found mainly in West Africa, and also in Central and South America. Most likely, this species was originally only in Africa, and was introduced to the Americas with African slaves. There are two strains of this species, distinguishable at the DNA level by the O-150 PCR test. One strain is typically found in the savanna regions of West Africa and the Americas, while the other strain is commonly found in rain forest areas. (Ogunrinade, et al., 1999; Roberts and Janovy, Jr, 2000)

Habitat

The habitat of Onchocerca volvulus is the body of its host. In the blackfly, microfilariae reside in the thoracic flight muscles. In humans, microfilariae can be found in the skin, lymph vessels, lymph nodes, blood, urine, cerebrospinal fluid, and internal organs (especially the eyes). Adults can be found knotted together in pairs or groups in the subcutaneous tissues. Due to host immunological reactions, these groups of worms often become encapsulated in fibrous tissue mainly composed of collagen. When these sites are located over bones such as a joint or skull, a conspicuous nodule called an onchocercoma appears, which is generally about 3 cm in diameter. Nodule location depends on geographical area, with most infections in Africa exhibiting nodules below the waist (especially on the knees and pelvic area), and those in Central America exhibiting nodules above the waist (especially on the neck and head). This relationship is based on the biting preferences of the blackfly vector, with African flies preferring to bite below the waist, and Central American flies preferring to bite above the waist.

Onchocerca volvulus will generally be found causing disease in savanna and rain forest areas, although occasionally it can be found in arid savannas and desert areas as well. (Burnham, 1998; Roberts and Janovy, Jr, 2000)

Physical Description

Adults of Onchocerca volvulus are long and slender, have a smooth cuticle, and have blunt anterior and posterior ends. There are no lips or buccal capsule, and the mouth is surrounded by 2 circles made up of four papillae each. The esophagus doesn't appear to be divided. Males range in length from 19-42 cm and range in width from 130-210 micrometers. Lacking alae, their tails are curled ventrally and bear 6 or 8 pairs of postanal and 4 pairs of adanal papillae. Females are larger than males, ranging in length from 33.5-50 cm and in width from 270-400 micrometers. Their vulva is directly behind the posterior end of the esophagus. The microfilariae released by adult females are 250-300 micrometers long, are unsheathed, and have sharply pointed and curved tails. (Burnham, 1998; Roberts and Janovy, Jr, 2000)

Development

Onchocerca volvulus has a complex life cycle involving a definitive and intermediate host. The only natural definitive host is humans. The intermediate host is a blackfly in the genus Simulium. Common blackfly hosts include Similium damnosum and S. neavei in Africa and S. ochraceum, S. metallicum, S. callidum, and S. exiguum in the Americas. Adult worms typically live from 8-15 years in human hosts, and microfilariae can live up to 2 years. Adults can be found grouped together in various regions of the human body, with groups containing about half as many males as females. The males move from group to group, inseminating the females. Fertilized eggs develop into live microfilariae in 3-12 weeks, and the females release the microfilariae into the host body. A single female worm may release 1300-1900 microfilariae per day for 9-11 years. The microfilariae migrate away from the adult worms, through blood vessels, lymphatics, skin and connective tissue, and generally settle in the skin.

Since the mandibles of the blackfly are not useful for deep piercing, a blackfly feeding on an infected human host will ingest blood and tissue fluid containing microfilariae. Once ingested, the microfilariae are attracted to the fly's thoracic flight muscles, and travel there. In 6-12 days, the microfilariae then develop into juvenile stage 1 (J1), molt into juvenile stage 2 (J2), and molt into juvenile stage 3 (J3), which is the infective, filariform stage. J3 worms then migrate to the labium of the blackfly host, and can infect new human hosts when the female fly next feeds on blood. Upon gaining entry into a human host, the J3 worms develop into adults in 1-3 months. After initial infection, microfilariae from the adults will appear in 10-20 months. (Burnham, 1998; Richards and Hopkins, 2000; Roberts and Janovy, Jr, 2000)

Reproduction

Adult worms typically live from 8-15 years in human hosts, and microfilariae can live up to 2 years. Adults can be found grouped together in various regions of the human body, with groups containing about half as many males as females. The males move from group to group, inseminating the females.

Females may produce a phermomone to attract males. The male coils around a female with his curved area over the female genital pore. The gubernaculum, made of cuticle tissue, guides spicules which extend through the cloaca and anus. Males use spicules to hold the female during copulation. Nematode sperm are amoeboid-like and lack flagella.

Communication and Perception

Nematodes in general have papillae, setae and amphids as the main sense organs. Setae detect motion (mechanoreceptors), while amphids detect chemicals (chemoreceptors). When mating, females may produce a pheromone to attract males. (Barnes, 1987)

Food Habits

Adults receive nutrients either from ingesting blood, or from absorption via a highly convoluted cuticle. It is common for rich vascular networks to develop surrounding groups of worms (the worms probably stimulate vessel formation), and these vessels bring the worms in close proximity with blood and nutrients. (Burnham, 1998)

Economic Importance for Humans: Positive

none

Economic Importance for Humans: Negative

Onchocerca volvulus is the pathogen responsible for the disease onchocerciasis, or river blindness. Infecting at least 17.7 million people worldwide, this non-fatal but severely debilitating disease is endemic in 37 countries (30 in Africa, 6 in the Americas, and Yemen). The pathology of onchocerciasis is based on host immune responses to the adult worm and the microfilariae, with the microfilariae being much more immunogenic. Little inflammation is caused by live microfilariae, but dead and degenerating microfilariae in the skin cause severe dermatitis, intense itching, skin depigmentation, skin thickening, skin discoloration, cracking of skin, and loss of skin elasticity. Quite often, microfilariae migrate to the eye, and their death causes intense inflammation. Over years this sclerosing (scarring) keratitis hardens the cornea and causes blindness. Worldwide, onchocerciasis impairs the vision of 500,000 people, and causes complete blindness in another 270,000. In some communities, the frequency of visual impairment is as high as 30%, and the frequency of blindness can be more than 10%. Additionally, elephantiasis may result (from microfilarial infestation of the lymphatic system), along with associated hernias. Furthermore, this disease results in decreased worker productivity, and the skin disfiguration can reduce marital prospects, reduce dowry size, and disrupt social relationships. (Burnham, 1998; Richards and Hopkins, 2000; Roberts and Janovy, Jr, 2000)

Conservation Status

Other Comments

Onchocerciasis is called "river blindness" because the intermediate host of Onchocerca volvulus, the blackfly, lives its egg, larval, and pupal stages in fast-running streams and rivers. This makes the flies most abundant near rivers, and makes areas near rivers the most likely place to get infected.

There are a number of methods for controlling onchocerciasis. First, there is the practice of excision of onchocercomas (especially those around the head) in order to reduce the rate of visual impairment and the number of new infections. Secondly, chemotherapy with the drug ivermectin has proved very successful. Donated in 1987 by Merck & Co., Inc. for mass treatment in developing nations, ivermectin is tolerated by humans, and a single oral dose given annually eliminates skin microfilariae, interrupts the transmission cycle by suppressing adult female microfilariae release, improves skin disease, and eventually kills adult worms. Thirdly, larvicides have been used to reduce the blackfly vector population. Programs such as the African Program for Onchocerciasis Control (APOC) and the West African Onchocerciasis Control Program (OCP) have used these control methods to combat the disease. (Hougard, et al., 1997; Molyneux and Davies, 1997; Richards and Hopkins, 2000; Roberts and Janovy, Jr, 2000)

Contributors

Glossary

Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

Neotropical

living in the southern part of the New World. In other words, Central and South America.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

causes disease in humans

an animal which directly causes disease in humans. For example, diseases caused by infection of filarial nematodes (elephantiasis and river blindness).

chemical

uses smells or other chemicals to communicate

desert or dunes

in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

fertilization

union of egg and spermatozoan

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body

marsh

marshes are wetland areas often dominated by grasses and reeds.

motile

having the capacity to move from one place to another.

ovoviviparous

reproduction in which eggs develop within the maternal body without additional nourishment from the parent and hatch within the parent or immediately after laying.

parasite

an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death

pheromones

chemicals released into air or water that are detected by and responded to by other animals of the same species

rainforest

rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.

riparian

Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

scrub forest

scrub forests develop in areas that experience dry seasons.

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

swamp

a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.

tactile

uses touch to communicate

terrestrial

Living on the ground.

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.

savanna

A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.

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The Animal Diversity Web is an educational resource written largely by and for college students. ADW doesn't cover all species in the world, nor does it include all the latest scientific information about organisms we describe. Though we edit our accounts for accuracy, we cannot guarantee all information in those accounts. While ADW staff and contributors provide references to books and websites that we believe are reputable, we cannot necessarily endorse the contents of references beyond our control.

This material is based upon work supported by the
National Science Foundation
Grants DRL 0089283, DRL 0628151, DUE 0633095, DRL 0918590, and DUE 1122742. Additional support has come from the Marisla Foundation, UM College of Literature, Science, and the Arts, Museum of Zoology, and Information and Technology Services.